Quality Decorative Plating

Article
From: Products Finishing, Dr. Donald Snyder
from
Atotech USA

Posted on:12/1/1996

Points to consider when looking for a quality decorative nickel finish...

Electroplaters, purchasers and designers using decorative electroplating realize that their parts must be of quality in order to compete in the global business market. Many organizations write and specify standards to ensure quality parts. The more detailed standards include final appearance, thickness and other tests that must be passed to demonstrate quality. These standards help obtain consistency in the electroplating operations, but they do not guarantee quality.

In addition to the physical characteristics of the electrodeposits, other important facts that must be included in a standard are part design, substrate specification and properties, surface preparation and preplate, and electroplating operations. Once specified within the standard, processes must be maintained within operating specifications to obtain a quality part.

Specifying physical properties. When developing a standard, it is best for the designer and standard writer to start by identifying the specific service life and environment the part will be subjected to. Field surveys of similar parts are one method. Surveys take time, but they are generally more reliable than laboratory tests in predicting usage data. Realistic accelerated tests, such as corrosion tests, are lacking in the decorative electroplating industry. Summary standards, such as those prepared by ASTM (American Society for Testing and Materials) are also good starting points.

With as much service life information as possible, the designer can specify the specific physical properties of the electrodeposits. Once an electrodeposit is specified in a standard, the electroplater must obtain processes that are capable of producing these properties. It is important to use an electroplating process that produces deposits with the required physical properties while operating within the middle of its operating range.

Narrow fluctuations around the center of the operating range are nearly always acceptable. With some processes, operating toward either end of the operating range may be acceptable if the physical properties are tested and fit the standards established for the product. For example, adjusting the bright nickel electroplating process to pass the STEP1 test is typically done by adding bright nickel additives to the process until the electrochemical potential for the bright nickel deposit is high enough to reach the minimum specified STEP value. The STEP requirement is obtained, but ductility, stress, adhesion, hardness and chromium receptivity are sacrificed.

Preplate requirements. For most substrates, there is information for designers and electroplaters to help determine the best way to prepare parts for electroplating. Substrates with defects such as holes, cracks, splinters and stressed surfaces will almost always lead to premature corrosion, appearance loss and even part failure.

Premature failure may also occur if a part is not cleaned and activated properly. Without a correctly prepared substrate, the electroplating will be unable to perform properly.

Acid copper electrodeposits. On decorative electroplated parts using nickel and chromium electrodeposits, acid copper deposits are occasionally used prior to nickel electroplating primarily to level over substrate defects such as holes, cracks, polishing lines and splinters. Acid copper produces an excellent substrate for nickel because of its ability to micro-throw into small irregularities in the substrate.

To obtain the best appearance for the part and substrate prior to nickel electroplating, copper deposits are polished and buffed. During polishing and buffing, the copper actually flows and fills defects in the substrate, eliminating them as potential corrosion sites. Copper electrodeposits have a high level of ductility that permits the part to withstand deformations, such as those encountered during thermal cycling. This makes it ideal for plastic substrates.

There is controversy concerning the amount of basis metal corrosion protection acid-copper deposits contribute. According to ASTM standards, in highly corrosive environments such as for decorative exterior automotive parts, a maximum of five micrometers of nickel can be replaced by 15 micrometers of copper.

One advantage of substituting copper for some nickel is that copper is less expensive than nickel. Since the better copper deposits level as well as or better than nickel, a thick copper deposit can be used to obtain more total leveling at about the same cost. On some parts, an increase in leveling could eliminate some substrate preparation steps and costs. Copper's ability to fill in defects more effectively than nickel also justifies substituting some copper for nickel.

There are two generic types of bright acid copper processes typically used for plating decorative deposits. The oldest technology uses a dye-type compound to enhance the deposit's leveling and appearance. Dye processes typically require the copper deposits to be acid activated prior to nickel plating to eliminate potential nickel exfoliation.

The other technology does not use a dye compound, making it safer to handle. This process also does not require an acid activator for the copper deposit prior to nickel plating. This makes the plating line shorter and easier to operate and maintain. It also eliminates one possible cause of nickel exfoliation.

There are some physical properties that may be considered when determining which acid copper is best for a part. Some copper deposits are easier to polish because they are softer when plated. There is also a difference in the ability of some processes to level and brighten low current density areas. These better appearing processes sometimes have a higher level of organics co-deposited in or adhering to the copper deposits. As the amount of organics associated with the copper deposit increase, the potential for poor nickel adhesion also increases.

Semi-bright nickel electrodeposits. For decorative plated parts that need a high level of basis metal corrosion protection, semi-bright nickel deposits are almost always used in conjunction with subsequent deposits of bright nickel and chromium. The semi-bright nickel deposit should be between 60 and 70 pct of the total nickel. This offers the highest level of basis metal corrosion protection with the lowest total nickel thickness and the best appearance.

Other than possibly the chromium deposit, the semi-bright nickel deposit is the most corrosion resistant barrier layer used on typical decorative plated parts. Its corrosion resistance is due to having an almost pure nickel composition with less than 0.005 mass pct sulfur co-deposited. The sulfur, an impurity, comes from the organic additives added to the plating process to produce a fine-grain, columnar, easy-to-buff nickel deposit.

Semi-bright nickel deposits are able to level out substrate defects such as polishing marks. These deposits can also be electroplated easily with bright nickel. Nickel activation and even rinsing is not required.

The most common semi-bright nickel processes use coumarin and formaldehyde as additives. These additives perform well, but they are classified as carcinogenic on some hazardous chemical lists. A few newer processes do not use these additives and produce deposits as good as deposits from the older technology. The newer processes are more sensitive to additive abuse, but they are easy to operate and require less frequent carbon purification.

Semi-bright nickel deposits can be dull gray to bright. The appearance depends on the process used, additives and current density. The color should be carefully monitored because it is one factor that can be used to maintain a consistent quality deposit. A color change could mean a change in the deposit's physical properties. Excess additives to enhance the low current density areas or increase leveling usually change the physical properties of the middle to high current density areas. Step plating, striations, stress cracking and reduced ductility are typical results of high levels of additives, especially in the non-coumarin process.

Careful monitoring of bath chemistry is critical when auxiliary anodes are used. The inert anodes preferentially deplete some of the compounds within nickel additives faster than others, making it more difficult to maintain the proper balance without changing the physical properties. This is also observed in bright nickel plating processes.

Bright nickel electrodeposits. There are distinctly different types of nickel plating processes. These are required in our modern quality market because each process produces nickel deposits with different physical properties and at various levels.

Bright nickel deposits perform three basic functions for decorative plated parts: appearance, corrosion resistance and performance. As its name implies, bright nickel deposits are typically bright, mirror-like, silver-colored reflective surfaces. The degree of brightness depends on the brightness of the deposit under the bright nickel, the thickness of the bright nickel deposit, the type of bright nickel process used and the actual (not average) current density at which it is applied.

Bright nickel deposits plate slower and consume additives at a different rate in the low current density areas of a part. A slower plating rate results in a thinner deposit. The lower plating rate is due to less current being available in the lower current density areas because the higher current density areas rob a disproportionate amount of current.

Reduced additive consumption occurs in areas of parts that receive less agitation. A high level of additive consumption can occur at low current density areas that have no restrictions on solution movement. The additives give the deposit its good physical properties such as brightness. The problem is that the physical properties of the deposits can also be negatively affected (dullness) at high and low additive levels. There are "fast brightening" processes that help produce better appearing deposits in low current density areas.

High- and low-potential bright nickel electrodeposits. There are two bright nickel plating processes based on their electrochemical potential. Except for the STEP test, these two classes have been divided based upon STEP values obtained when the bright nickel deposit is plated over a well-operated coumarin-based semi-bright nickel deposit. Processes that produce STEP values in the 120 to 140 mv range or lower are low-potential bright nickel deposits. Bright nickel deposits with values around 150 to 180 mv or higher are high-potential processes. Visually, the deposits cannot be distinguished except after corrosion.

In duplex nickel systems, where semi-bright nickel is deposited prior to the bright nickel, the basis metal or substrate is protected from corrosion by the electrochemical interaction of the two nickel deposits. Because the deposits are more electrochemically active than the semi-bright nickel deposits, the bright nickel deposit sacrificially protects the semi-bright deposit. The greater the difference in potential, the more sacrificial the bright nickel.

As the bright nickel sacrificially corrodes to protect the semi-bright nickel, the bright nickel deposit dissolves leaving a surface pit. The higher the potential difference between the two nickel deposits, the larger the surface pit will be. Because of this, a duplex nickel system with a high-potential nickel deposit will protect the semi-bright nickel and basis metal longer from corrosion.

For duplex nickel systems, there is a compromise between improved basis metal corrosion protection and improved appearance after corrosion. To obtain the best of one, the other must suffer. If basis metal protection is important, the high potential nickel plating system is preferred, when using duplex nickel systems.

Substrates such as iron/steel are more electrochemically active than even the high-potential bright nickel deposits. When these substrates are exposed to the corrosion processes, the substrate preferentially corrodes, protecting the nickel. From a decorative viewpoint, the tenacious red rust corrosion products are far more objectionable than surface pitting.

When only bright nickel deposits are plated over the substrate or electroplated copper, the low-potential bright nickel processes are usually preferred. Then high-potential bright nickel corrodes a little faster than low-potential bright nickel deposits, protecting the basis metal longer.

Chromium electrodeposits. Two different chemistries for plating decorative chromium deposits are hexavalent and trivalent. Hexavalent is the oldest chemistry. Its ions have been identified as carcinogenic, toxic and are becoming more regulated, affecting a plater's waste treatment and air and water discharges.

Trivalent chromium has not been identified as carcinogenic. There are operational advantages for the trivalent chromium process and a few differences in the physical properties of the deposits.

Chromium deposits are applied to decorative nickel deposits for their physical properties. Chromium deposits do not corrode under normal environmental conditions due to the chromium oxide coating. The deposit also imparts lubricity and wear properties. They enhance long-term decorative appearance of parts plated with nickel and chromium electrodeposits.

Although there are a variety of hexavalent chromium baths, the physical properties of the deposit are similar. Hexavalent chromium deposits are either microporous or microcracked. Microdiscontinuous chromium deposits spread the corrosion reaction out over millions of small galvanic corrosion sites, reducing the rate of corrosion at each site. Most standards require 10 to 17 thousand pores per sq cm or 40 microcracks per linear inch. As the corrosion develops, the nickel at the corrosion site is corroded away, and the thin chromium deposit over the hole collapses. These invisible holes in the chromium deposit before corrosion becomes visible. The total nickel thickness on plated parts that are subjected to a highly corrosive environment can be reduced when microdiscontinuous chromium deposits are used over duplex nickel.

Most decorative electroplated chromium deposits are thin. For the most corrosive environment, such as exterior automotive decorative parts, a minimum of ten millionths of an inch is specified. Thicker deposits, such as 25 millionths of an inch, will macrocrack. Macro-cracked chromium is not decorative in appearance and its corrosion protection is greatly diminished.

Chromium deposits from trivalent baths can be plated to about 50 millionths of an inch before macrocracking. Since the plating rate is about two to three times faster than most hexavalent chromium processes, deposits of 20 to 25 millionth of an inch can be obtained in the same plating time as used for hexavalent chromium.

Trivalent chromium deposits are also microdiscontinuous as plated. This gives parts the improved corrosion resistance properties that can only be obtained from hexavalent chromium baths after special operations.

Depending on the process, trivalent chromium deposits vary in color from a stainless steel or pewter to the color of deposits from hexavalent chromium. Deposits from hexavalent chromium electrolytes have a slight blue color that trivalent chromium produced deposits do not have.

Quality parts are obtainable with the present level of knowledge and installed electroplating operations. For those parts that do not have the required quality, standards can be written that specify the deposits and the physical properties that are required to obtain the intended performance. PF

Exerpted from a paper presented at Qualifinish '93, March 23-25, 1993, Cincinnati, Ohio.

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